1. Field of the Invention
The present invention relates to virtual local-area-network (VLAN) servers, and more particularly, to a physical VLAN server connected to a network which employs TCP/IP and capable of providing services for a plurality of VLANs by itself alone.
2. Description of the Related Art
A virtual local area network (VLAN) refers to a technology for making a virtual group by terminals connected to a LAN irrespective of their physical connections.
A port VLAN refers to a system in which a port number is assigned to each VLAN in advance in a concentrator (hereinafter called a switching hub) installed in a LAN, a virtual group is formed for the port which each VLAN uses, and communications are made. A tag VLAN (according to the IEEE 802.1Q standard) refers to a system in which a tag (a predetermined value) is assigned to each VLAN in advance in a switching hub constituting a network, a virtual group is formed for the tag of data handled by each VLAN, and communications are made.
The value of the tag attached to a media-access-control (MAC) frame is used to identify the VLAN to which each terminal belongs. VLANs provide advantages. A LAN can be divided in order to reduce the amount of line traffic. A plurality of VLANs can share physical wiring to reduce cost. Since VLANs are easy to operate and administrate, they have been rapidly spreading to various types of corporations.
When a service usually provided by one server, such as a web application and a file transfer function, is provided for VLANs, it is disadvantageous in terms of cost and in terms of operation and administration to install one server in each VLAN.
To avoid this disadvantage, methods have been proposed in which one physical server is shared among VLANs. In such methods, it is suitable that the same number of identical application (such as web browsing or file transfer protocol (ftp)) processes (hereinafter called server processes) as the number of the VLANs are activated in the server. Such a server is provided, for example, by Japanese Unexamined Patent Publication No. 2003-167805.
In Japanese Unexamined Patent Publication No. 2003-167805, a server uses a VLAN tag to identify the closed network to which a communication destination client belongs. More specifically, a VLAN tag is attached to an MAC frame sent from a client, and therefore, the server can identify the VLAN to which the transmission source client of the frame belongs, by the use of the VLAN tag of a received frame. As a network protocol for transferring the received MAC frame to a server process, TCP/IP is used. The same number of TCP/IP stacks and the same number of device drivers as the number of accommodated closed networks are provided as a feature in Japanese Unexamined Patent Publication No. 2003-167805. In other words, each server process can communicate with a certain closed network by receiving data from the TCP/IP stack corresponding to the closed network.
In Japanese Unexamined Patent Publication No. 2003-167805, however, since a plurality of TCP/IP stacks and a plurality of device drivers are provided, a memory needs to have the same number of data items of each type, such as a routing table for determining a path to a communication destination and an address-resolution-protocol (ARP) table for managing the correspondence between an IP address and a MAC address, as the number of closed networks, which may cause an increase in the amount of used memory. In addition, in Japanese Unexamined Patent Publication No. 2003-167805, each of the communication flows for closed networks A to C corresponds to one computer, and therefore, the same number of computers as the number of the closed networks are required, which may cause an increase in used hardware.